Combined cycle power plants are plants that produce electrical power via generators driven by gas and steam turbines. The gas turbine is connected to an electrical generator to produce electrical power. Also, high grade heat in the flue gas from the gas turbine exhaust can be directed to a heat recovery steam generator (HRSG) to generate steam. This steam is used as a driver for a steam turbine that is connected to another generator to produce additional electrical power as shown in FIG. 1. The combined cycle includes two cycles working together in most cases. The gas turbine can use a Brayton cycle, and a Rankine cycle can be used for steam generation. HRSG is a major part of the combined cycle that heavily contributes to the cycle efficiency.
A HRSG 100 is used to convert waste heat energy from gas turbine exhaust 101 exhausted from gas turbine 104. The gas turbine exhaust is used to superheat steam that is used to drive steam turbines, namely high pressure steam turbine 102a and low pressure steam turbine 102b for the purpose of power generation. The HRSG 100 has three main sets of tube banks; economizers 104a, 104b, evaporators 105a, 105b and super heaters 106a, 106b that are heated via the gas turbine exhaust 101. The function of the economizers 104a, 104b is to raise water temperature to its saturation point, while evaporators 105a, 105b produce saturated steam. Superheated steam generated in the superheaters 106a, 106b will then operate the steam turbines 102a and 102b respectively. The tube banks can be found in single or multiple pressure stages. The HRSG 100 is treated as a boiler in terms of material selection and design.
There are two methods for circulating the steam inside the HRSG 100. FIG. 2a shows a method for naturally circulating steam inside of the HRSG 100. Natural circulation of the water vapor mixture is normally used for horizontal gas flow HRSGs. However, due to footprint limitations, induced flow circulation via additional pumps may be used in a vertical gas flow arrangement as shown in FIG. 2b to circulate the water-steam mixture through horizontal tubes.
HRSGs used in combined cycle plants typically operate and rely on the gas turbine exhaust rather than using auxiliary firing. However, in cogeneration plants, a complementary furnace or auxiliary firing system may be implemented to further increase the steam generation.
HRSGs use a flue gas from turbine exhaust to produce superheated steam to operate a steam turbine in combined cycle power plants. This flue gas passes through a duct, such as duct 100 before it impinges on the super heater tube banks. The exhaust from a gas turbine is extremely turbulent having a high temperature and speed. The exhaust gas from an exhaust gas turbine traverses the duct 300 having a duct inlet portion 301 that expands at the duct expansion portion 302 as shown in FIG. 3. As the exhaust gas passes from the duct inlet portion 301 to the duct expansion portion 302 the sudden expansion in the duct 300 at the expansion angle 303 causes flow separation and a reverse flow at the exit of the duct expansion portion 302. The flow separation and reverse flow disadvantageously subjects the tube banks to hot spots and causes a significant reduction in HRSG efficiency.